<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <title>Document</title>
</head>
<style>
    .box {
        width: 500px;
        height: 400px;
        border: 1px solid #000;
        position: relative;
        margin: auto;
        overflow: hidden;
        cursor: pointer;
    }

    .box ul,
    .box ol {
        list-style: none;
        padding: 0;
        margin: 0;
    }

    .box ul li {
        width: 500px;
        height: 400px;
        display: none;
    }

    .box ul li img {
        width: 500px;
        height: 400px;
    }

    .box ol {
        width: 120px;
        position: absolute;
        left: 50%;
        margin-left: -40px;
        bottom: 20px;
        height: 20px;
        background: #fff;
        display: flex;
        justify-content: space-evenly;
        padding: 10px 0;
        border-radius: 20px;
    }

    .box ol li {
        width: 20px;
        height: 20px;
        border-radius: 50%;
        background: #f00;
        border: 1px solid #999;
    }

    .box ol li.current {
        background: #00f;
    }

    .box ul li.current {
        display: block;
    }

    .box .left,
    .box .right {
        position: absolute;
        top: 50%;
        width: 30px;
        height: 30px;
        margin-top: -15px;
        background: #ccc;
        text-align: center;
        line-height: 30px;
        text-decoration: none;
        font-weight: bold;
        color: #fff;
    }

    .left {
        left: 0;
    }

    .right {
        right: 0;
    }
</style>

<body>
    <div class="box">
        <ul>
            <li class="current"><img src="./img/5.jpg" alt=""></li>
            <li><img src="./img/1.jpg" alt=""></li>
            <li><img src="./img/3.jpg" alt=""></li>
            <li><img src="./img/4.png" alt=""></li>
        </ul>
        <ol>
            <li class="current"></li>
            <li></li>
            <li></li>
            <li></li>
        </ol>
        <a href="javascript:;" class="left">&lt;</a>
        <a href="javascript:;" class="right">&gt;</a>
    </div>
</body>
<script type="text/javascript">
    /*
        实现功能
        1、自动切换图片 保证小圆点跟随
        2、上一张下一张点击切换图片  小圆点也需要跟随
        3、点击小圆点 切换到对应的图片
    */
    //定时器里ul li display
    var index = 0;
    var timer;
    var imgDom = document.querySelectorAll('ul>li');
    var cirDom = document.querySelectorAll('ol>li');
    var aLeft = document.querySelector('.left');
    var aRight = document.querySelector('.right');
    auto();

    function auto(){
        timer = setInterval(function () {
        var ulCurrent = document.querySelector('ul>.current');
        var olCurrent = document.querySelector('ol>.current');
        index++;
        if (ulCurrent && olCurrent) {
            ulCurrent.className = '';
            olCurrent.className = '';
        }
        if (index > 3) {
            index = 0;
        }
        imgDom[index].setAttribute('class', 'current');
        cirDom[index].setAttribute('class', 'current');
      
    }, 1000)
    }

document.querySelector('ul').addEventListener('mouseover',function(){
    clearInterval(timer);
    aLeft.addEventListener('click',function(){
        clearInterval(timer);
        if(index ==0){
            index=4;
        }
        index--;
        console.log(index);
        if(document.querySelector('ul>.current') && document.querySelector('ol>.current')){
            document.querySelector('ul>.current').className='';
            document.querySelector('ol>.current').className='';
        }

        imgDom[index].setAttribute('class', 'current');
        cirDom[index].setAttribute('class', 'current');
    })
})


document.querySelector('.left').addEventListener('mouseout',auto);
    //点击 a 标签left 清除定时器，找到当前img，this 上一个兄弟元素
</script>

</html>